Demand cardiac pacemaker and method



United States Patent Inventors Appl. No. Filed Patented Assignee DEMAND CARDIAC PACEMAKER AND METHOD [56] References Cited UNITED STATES PATENTS 3,345,990 /1967 Berkovits l28/4l9 Primary Examiner William E. Kamm Attorney- A. C. Smith ABSTRACT: The method and means for demand pacing the 5 Cl 2 D heart of a patientinhibits beat synchronization from artificially rawmg stimulated heartbeats so that the heart is correctly stimulated 128/422 at not less than a preselected beat rate during intermittent or A6lh 31/00 continuous failure of natural rhythm. The beat-stimulating 128/419- pulses applied to the patient are monitored to provide an indi- 424, 4 l 9P digest cation ofdrop in pulse magnitude below a preselected level.

F l 5 7 1s 17 l i6 21 8 i r l V f r Y [El RATE PACE PULSE l sumrmc ms PULSE flgig'gh fl csuemon BUFFER mpiinen I Z l 25 9 29 19 I l r ms o Ecc rm: I cmcun cgr gg on 1 OUTPUT MONOSTABLE METER MULTIVIBRATOR INDICATOR DEMAND CARDIAC PACEMAKER AND METHOD BACKGROUND OF THE INVENTION Certain known demand cardiac pacers are inhibited from producing a beat-stimulating pulse in response to the appearance of a natural beat within a preselected interval following the preceding beat. The next beat interval is thus synchronized to the QRS portion of the electrocardial signal of the heartbeat thus stimulated. However, in these known pacers, this next beat interval is increased by the time delay occurring between the beat-stimulating signal and the resulting heartbeat, thereby producing a lower heartbeat rate per unit time than the preset rate of a beat per selected time interval. Also, the beat-stimulating signal applied to the heart of a patient may decrease in magnitude below a selected value due to pacing electrode contact problems, or the like, and become insufficient to reliably stimulate the patients heartbeat.

SUMMARY OF THE INVENTION Accordingly, the cardiac pacing apparatus and method of the present invention provides heartbeat-stimulating signals in response to the absence of natural heartbeat within a preselected time interval which is not altered by the appearance of a stimulated heartbeat signal.

DESCRIPTION OF THE DRAWING FIGURE 1 is a block schematic diagram of the cardiac pacing circuit of the present invention; and

FIGURES 2a and b are graphs on a common time axis of the relationships of the cardiac signal and heartbeat-stimulating signals.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGURE I there is shown a rate generator 5 which triggers pace pulse generator 7 at a pulse rate which is selected by the rate control 8 of the rate generator. The electro-cardiograph (ECG) circuit 9 which is connected to receive electro-cardial signals from a patient produces inhibit pulses on line ll representative of the occurrence of a QRS portion of an electro-cardial signal from a patient. An inhibit pulse appearing on line 11 due to a natural heartbeat occurring prior to the end of the normal period of rate generator 5 resets the rate generator 5 to the start of a new pulse period and thereby inhibits rate generator 5 from triggering the pace pulse generator 7 as a result of the appearance of the natural heartbeat. Thus no stimulating pace pulses are produced during normal heart activity which produces beats at a rate faster than the preset slower pacing rate.

When no heartbeat occurs naturally prior to the end of the preset pacing period, rate generator 5 triggers pulse generator 7 and activates the monostable multivibrator 13. The pace pulse generator 7 supplies a pace pulse to the amplifier 16 including buffer and amplifier stages [5 and 17 which, together with the pace current control 19 in the return path, constitute a negative-feedback stabilized source of pulse current having a value set by the pace current control l9. This amplified pulse isapplied to the primary winding of an output transformer 21 to produce the desired heartbeat-stimulating pulse 23, as shown in FIGURE 2a, at the output terminals 25 which are connected to the secondary of the output transformer 21. The output terminals 25 may be connected to apply the pulses 23 directly to the myocardium of the patient, as by suitable catheters, or the like. The output terminals 25 may also be connected to the chest wall by suitable fluid column electrodes, or the like. Transformer 21 provides electrical isolation of the patient from the circuitry of the present invention.

At the same time the rate generator 5 triggers the pace pulse generator 7 it also triggers the monostable multivibrator 13. This multivibrator produces a disabling signal on line 27 for a selected period t that is longer than the time interval which normally exists between the pace pulse 23 and the resulting stimulated QRS portion of the electrocardial signal from the patient. This disabling signal prevents the rate generator reset 29'from resetting on the stimulated heartbeat and thereby delaying a subsequent pace pulse by the preset pulse interval plus the delay interval, as shown in the graph of FIGURE 2b. Instead, the rate generator 5 triggers the pace pulse generator 7 after each t time interval independent of the occurrence of a stimulated QRS pulse at a delayed time t,, after application of the pace pulse 23. The stimulated pulse rate is thus accurately set at a pulse per time interval t and is not affected by the time delay t normally occurring between the pace pulse and the stimulated QRS portion of the patients electrocardial signal.

The monostable multivibrator 13 also applies a signal to meter driver 30 which, in turn, produces an indication on meter 31 representative of the pace pulse rate established by the rate generator 5. Also, the output impedance detector 33 is connected in the primary circuit of the transformer 21 to detect the current level of pace pulses 23 delivered to a patient. Detector 33 responds to the voltage across the primary winding of the output transformer 21 for determining the impedance presented to the secondary of transformer 21 by the patient during the beat stimulating pulses. This information may be used to provide an indication of proper conduction at electrode-tissue interface, and the like, rather than a mere indication of the formation of a pace pulse, so that appropriate and timely corrections may be made to insure continued conduction of adequate current through the patient for proper heartbeat stimulation. The actual current delivered to the patient with each pace pulse 23, as determined by the setting of pace current control 19, may also be indicated by the indicator device 35 which may, for example, include an audible or visual alarm or a meter. Where alarms are used, the indicator 35 may include a level detector which is responsive to the setting of control 19 to provide an alarm only when the pulse current delivered to the patient drops below a selected value determined by the setting of control 19.

Therefore, the cardiac pacer of the present invention sup-' plies heartbeat-stimulating pulses to a patient only upon failure of the natural rhythm. The stimulating pulses thus supplied recur at a preselected rate which is unaltered by the occurrence of a stimulated heartbeat that might otherwise be detected to be a natural beat. Also, the present invention pro vides an indication responsive to the pace pulse current con-' ducted by the patient rather than a mere indication of the formation of a pace pulse so that electrode conduction and the like may be checked upon each delivery of a pace pulse.

We claim:

1. Heart pacing apparatus comprising: I

heart-stimulating pulse-generating means for normally producing pulses at preselected intervals;

electrode means connected to said pulse-generating means for supplying heart-stimulating electrical pulses to a pa-' tient;

monitoring means connected to a patient to monitor the beating action of such patients heart;

reset means connected to said monitoring means and to said generating means and responsive to said monitoring means detecting a heartbeat of a patient within a preselected interval for resetting said generating means and initiating a new preselected interval and preventing generation of a heart-stimulating electrical pulse; and circuit means responsive to an electrical pulse delivered to said electrode means at the end of a preselected interval for inhibiting the resetting of said generating means to initiate a new preselected interval in response to said monitoring means detecting a heartbeat of a patient stimulated by said electrical pulse delivered to said electrode means.

2. Heart pacing apparatus as in Claim 1 wherein:

said circuit means includes delay means for producing an inhibit signal having a period which is longer than the period normally occurring between the application of a heartbeat-stimulating electrical pulse to a patient and the resulting stimulated heartbeat of such patient.

3. Heart pacing apparatus as in Claim 1 wherein:

said monitoring means includes means responsive to the electro-cardial signals of a patient and said circuit means includes a monostable multivibrator which is triggered upon delivery of an electrical pulse;

said resetting means is rendered inoperative by the output of said multivibrator to activate the pulse generating means to initiate a new preselected interval at the occurrence ofa heartbeat detected by said monitoring means.

4. Heart pacing apparatus comprising:

heartbeat-stimulating pulse generating means including means for setting the electrical pulse current at a preselected value;

an output transformer having a secondary winding and having a primary winding connected to receive pulses from said pulse generating means;

electrode means connected to the secondary winding of said output transformer for applying heartbeat-stimulating electrical pulses to a patient; and

circuit means connected to the primary winding of said output transformer for producing an indication in response to the electrical pulse current supplied to a patient decreasing below a level related to said preselected value 5. The method of pacing the heart of a patient comprising the steps of:

normally supplying heartbeat-stimulating electrical pulses to a patient at the ends of preselected intervals;

detecting the heartbeat of a patient and, in response to a natural heartbeat occurring within a preselected interval, initiating a new preselected interval; and

in response to application of a heartbeat-stimulating electrical pulse to a patient, inhibiting initiation of a new preselected interval in response to detection of the heartbeat stimulated by such electrical pulse. 

